Why your second job is taxed differently
Your tax-free personal allowance (£12,570 a year) is normally given to one job, usually your main one. Your second job then gets tax code BR, which means 20% tax on everything you earn there. That's not extra tax — it's how your allowance is split.
If your main job pays less than £12,570 a year, part of your allowance is wasted. You can ask HMRC to move some of it to your second job.
National Insurance works per job
National Insurance is worked out separately for each job, on each pay period. If you earn under the threshold (about £242 a week) in a job, you pay no NI on that job, even if your total across both jobs is higher.
This is an estimate
The calculator uses 2026/27 rates for England, Wales and Northern Ireland, an even monthly income and no pension or student loan deductions. Scotland has different income tax bands. Check your actual tax code on your payslip or in your HMRC app.